(1) Field of the Invention
During the study of fresh limonene as a hand cleaner, the applicants found that fresh limonene is an excellent solvent, but it is not bactericidal. Conversely, limonene which has undergone auto-oxidation, unexpectedy, was found to have a minimal, inconsistent antibacterial activity which varied in potency from batch to batch. Therefore, it was decided to purposely oxidize limonene to try to enhance and stabilize the antimicrobial properties of the limonene.
The applicants found that by bubbling air (or oxygen) to limonene to purposely oxidize (or oxygenate limonene which is the same) the oil, that it became bactericidal at progressively decreasing concentrations until it reached a point beyond which further oxidation caused a decrease in its bactericidal activity. As the purposeful oxidation of limonene progressed, it was noted that the physical properties of the limonene changed as its antimicrobial activity increased. Limonene, which was purposely oxidized was found to be effective in killing bacteria, yeasts, and fungi and its antimicrobial activity persisted unchanged when the purposely oxidized limonene was stored at 2.degree.-4.degree. C. in the dark, for periods exceeding one year.
The ability to produce limonene with a stable antimicrobial activity was unexpected and is not appreciated nor delineated in the prior art. In fact it is recognized in the prior art that the addition of air (or oxygen) to terpenes produces volatile mixtures of oxides and peroxides that are unstable. Limonene which undergoes auto-oxidation has an inconsistent antibacterial effect that is not rapidly bactericidal. It evaporates to a thick, non-usable gel before it becomes an effective antimicrobial when auto-oxidation is allowed to occur over a large surface area of the oil. On the other hand, limonene that is purposely oxidized becomes an excellent, stable bactericide and fungicidal that is bactericidal and fungicidal in bactericidal and fungicidal concentrations and its activity can be demonstrated with even short interval application.
Thus, the object of this invention is to economically produce oxidized limonene: (1) with a potent, rapidly acting antimicrobial activity, (2) that will retain its antimicrobial activity when stored, and (3) that is superior in its antimicrobial activity to the antimicrobial activity of auto-oxidized limonene.
Limonene is a terpene with the following formula: ##STR1##
Limonene is an oil with a terpenic aroma. It is soluble in alcohol and miscible in oil, but it is insoluble in glycerine. Limonene is usually derived from either a pine or citrus origin. The l-isomer is derived from pine and has a characteristic pine aroma while the d-isomer is derived from citrus and has a characteristic citrus aroma. The l-isomer is also made synthetically, but the synthetic l-limonene has no characteristic aroma.
Commercial limonene is usually obtained from the peel of citrus fruit. Pressed liquor is obtained from the peel of any citrus fruit, but most commonly from oranges and grapefruit, after which it is passed through a flash evaporation at 240.degree. F. at atmospheric conditions. Limonene is collected from the condensation of the flash vaporization. Limonene is used commercially as a flavoring agent, is used in large quantities to produce carvone which has a spearmint flavor, is used as a solvent, can be used as a base for soap or perfume, is used in the manufacture of rubber, and is useful as a penetrating oil.
Limonene undergoes auto-oxidation when it is exposed to air or oxygen. After exposure to air at room temperature, limonene becomes rancid with a foul smelling odor. The temperature has to be above about 40.degree. F. for auto-oxidation to occur while temperatures above 90.degree. F. increase the rate of evaporation. Because auto-oxidation limits the commercial value of limonene by causing it to become rancid with a foul smelling odor, limonene is usually stored in 55 gallon drums that have their interior painted with phenolic resin base enamel especially formulated for terpenes. The drums are completely filled and capped in order to limit exposure to air (and oxygen) that causes auto-oxidation. Auto-oxidized limonene develops antibacterial activity during auto-oxidation but does not become a rapidly acting bactericide. Auto-oxidation does not produce the potent, rapidly bactericidal limonene described in this patent.
(2) Description of the Prior Art
The oxidation of limonene produces several chemical compounds that have been enumerated by several different investigators. For instance Bain (in U.S. Pat. Nos. 2,863,882 and 3,014,047) showed a method of producing and recovering terpenic oxidation products in which he delineated the oxidation products of limonene as: limonene-1,2-oxide, limonene-8,9-oxide, 1-menthene-9-ol, .alpha.-2,8-p-menthadiene-1-ol, .beta.-2,8-p-menthadiene-1-ol, dihydrocarvone, .beta.-cymenol, carvone, cis-carveol, and trans-carveol. He never studied the antimicrobial activity of oxidized limonene nor oxidized limonene to the point that it became stable with a potent antimicrobial activity. In fact, because limonene is chemically unstable during its oxidation, Bain thought that in order to obtain the maximum number of oxidation products from limonene, that he should add alkali to limonene when the peroxide value was between 1000 and 2000 and should cease adding air or oxygen to limonene. He thought the peroxide value was the sole indicator of the formation of limonene oxidation products (oxides and peroxides) and that these products could be obtained only when the oxidation of limonene was stopped as the peroxide value rose to its maximum. He never realized that the maximal activity achievable through the oxidation of limonene is obtained before maximum peroxide formation occurs.
Blumann listed the compounds formed by the auto-oxidation of limonene in Chemical Abstracts, Volume 63, 1965, on page 1819, and found minute amounts of cis and trans-carveol, trans-p-menth-8-ene-1,2-diol, limonene 1,2-epoxide, limonene 8,9-epoxide, cis and trans-p-mentha-2,8-dien-1-ol, and perillyl alcohol, but he never tried to purposely oxidize limonene and never tried to develop limonene with a stable, rapidly bactericidal activity.
Bardyshev studied the chemical compounds in photo-initiated, auto-oxidized, 1-limonene and found tiny amounts of the following compounds: carvone, carveol, trans-p-menth-8-ene-1,2,-diol, hydroperoxide, p-menthane, p-cymene, terpinolene, p-mentha-2,4(8)-diene, cyclohexene, cuminic aldehyde, dihydrocarvone, piperitone, p-mentha-2,8-dien-1-ol, pulegone, carvomenthone, PhCOMe, 4-MeC.sub.6 H.sub.4 COMe, cyclohexanone, perillyl alcohol, and p-CH.sub.2 :CMeC.sub.6 H.sub.4 CH.sub.2 OH as was outlined in Chemical Abstracts, Volume 80, 1974, page 359. Like Blumann, Bardyshev never purposely oxidized limonene and never studied the antimicrobial activity of fresh nor auto-oxidized limonene.
Zuckerman studied the effect of d-limonene on bacteria and found that fresh limonene had no effect on bacteria. He found that when limonene became auto-oxidized, it developed minimal, inconsistent, weakly inhibitory (not bactericidal) properties. He noted that, at best, auto-oxidized limonene was only weakly inhibitory in that it only "shifts the time of the lag phase of bacterial growth without any influence on the logarithmic phase of bacterial growth." He noted that auto-oxidized limonene is unstable, and loses its inhibitory effect on keeping as was discussed in Nature 168: 517 (1951). He also recognized that the addition of alkali, as advocated by Bain in his method of oxidizing limonene, destroys the bacteriostatic properties of auto-oxidized limonene. He never recognized that limonene can be purposely oxidized to a point that it becomes a potent, stable antimicrobial which retains its antimicrobial activity when stored. In short, he could not make oxidized limonene with a stable, rapidly bactericidal activity.
In U.S. Pat. No. 3,595,975 Gauvreau showed a means of producing disinfecting compositions by combining cetyl pyridinium with terpenes to form antiseptics, but he never learned that limonene which had been purposely oxidized, becomes a potent, stable bactericide and fungicide. The active ingredient in his disinfecting compositions was cetyl pyridinium chloride (and not the terpenes).